Focus on Energy: Roof Venting How Much Is Enough?

Most builders in the snowy North
take pains to ventilate their roof systems,
even when it's not easy (see
"Venting Details for Cathedral
Ceilings," page 36). One important
reason is to prevent the formation of
damaging and dangerous ice at the
eaves. Deep snow insulates a roof and
traps heat inside the attic, but a wellvented
attic will allow the heat out
before it can melt the snow.
So how can a person determine how
much venting it takes to keep a given
roof icefree? Code requirements like the
1:300 rule (one square foot of free vent
area per 300 square feet of attic floor
area) and the 1:150 rule are a good
start, but in some